1. Field of the Invention
The present invention relates to a semiconductor wafer heating device, and more particularly, to a semiconductor wafer heating apparatus employed in vapor phase growth, oxidation, diffusion or annealing processes in the semiconductor manufacturing art.
2. Description of the Related Art
FIG. 1 is a schematic view of a vapor phase growth apparatus with a conventional semiconductor wafer heating device incorporated therein. In this apparatus, a semiconductor wafer placed on a heating stage 2 is heated. Reaction gas is injected toward the surface of the semiconductor wafer 1 from a gas head 3 while the semiconductor wafer 1 is being heated. A thin film is formed on the surface of the semiconductor wafer 1 by virtue of the thermal chemical reaction of the reaction gas. The non-reacted gas remaining in the apparatus is discharged to the outside of the apparatus from an exhaust port 5 of an exhaust chamber 4 disposed surrounding the gas head 3 and the heating stage 2. In the above-described apparatus, since the film growing rate is dependent on the temperature of the semiconductor wafer 1, the temperature of the latter, i.e., the surface temperature of the heating stage 2, must be maintained at a uniform value during the reaction in order to ensure that a uniform film will grow on the semiconductor wafer 1.
FIG. 2 schematically shows the structure of the conventional heating stage 2. In order to improve the uniformity of the temperature on a surface 2a of the heating stage 2, a heat equalizing plate 6 made of a material having a high coefficient of thermal conductivity is used as the heating stage 2, and a heater 7 is disposed uniformly over the rear surface of the heat equalizing plate 6, as shown in FIG. 3. Further, the heat equalizing plate 6 and the heater 7 are supported by means of supports 8 made of a material having a low coefficient of thermal conductivity to prevent escape of the heat. The heater 7 is connected to a power source 10 through terminals 9, and a thermo-couple 11 is inserted into the equalizing plate 6 from the rear surface thereof for measuring the temperature of the vicinity of the surface of the heat equalizing plate 6, the measured temperature being used for temperature control of the heating stage.
In the above-described type of semiconductor wafer heating device, attempts are made to make the heat emanating from the heater 7 uniform by virtue of the heat conduction. There is therefore a limit to the uniformity of the temperature of the heater 7. More specifically, heat escapes from the heater 7 uniformly disposed in a helical fashion, as shown in FIG. 3, through the supports 8 and the terminals 9, making the temperature of the heater 7 non-uniform. This problem involving the non-uniformity of the temperature of the heater 7 may be solved by employing a heat equalizing plate 6 having a substantial thickness t. However, such a thick heat equalizing plate 6 increases the heat retaining capacity of the overall heating stage 2 making control of the surface temperature of the semiconductor wafer 1 difficult. That is, the thick heat equalizing plate 6 makes it difficult for the surface temperature of the semiconductor wafer 1 to be maintained constant when it is subjected to disturbances which are represented by changes in the ambient temperature.
FIGS. 4 (a) and 4 (b) are respectively graphs showing the difficulty involved in controlling the temperature of the heating stage 2 when it is subjected to a disturbance indicated by A on the basis of the relationship between time and the surface temperature of the heating stage 2. FIG. 4 (a) illustrates a case in which a heating stage 2 comprises a thin heat equalizing plate 6 (having a small heat retaining capacity) and FIG. 4 (b) explains a case in which a heating stage 2 of a thick heat equalizing plate 6 (having a large heat retaining capacity) is used. It is, however, to be noted that in both cases the heater 7 has the same capacity. As will be seen from the graphs, in the case of a large heat retaining capacity, there is a time lag between the heating of the heater 7 and the heating of the surface of the heating stage 2, thus delaying the temperature recovery. It also increases the possibility that the phenomenon called overshoot may occur, the phenomenon representing the tendency for periodic deviation of the temperature of the heating stage 2 from the set value.
Thus, in the conventional semiconductor wafer heating device, only the effect of heat conduction is utilized to make the surface temperature of the heating stage 2 uniform. This requires a thick heat equalizing plate 6 and makes control of the temperature thereof difficult.